Researchers Go With the Floe To Unlock Secrets of Arctic Heat

SHEBA ICE STATION, ARCTIC OCEAN
— Like clockwork, an electric winch pays out 150 meters of cable down through a small hole in the ice, reels it back, then begins the process again.

With each descent, sensors at the cable's business end send back information designed to help scientists measure the flow of heat from the ocean to the floe. Remote buoys gather similar data about the heat flux between the open ocean and atmosphere.

Together, these sensors form a critical part of the SHEBA project, which for the next 13 months will gather detailed information on how heat is exchanged between the ocean, ice, and atmosphere during an Arctic year.

Collectively, the experiments that make up SHEBA are designed to provide a reality check on detailed computer programs that simulate the interactions between solar radiation, the oceans, and the atmosphere, which determine Earth's climate.

These computer models are used to anticipate the likely effects of global warming. One weakness, however, is the models' inability to accurately mimic - and forecast - conditions in the Arctic, a key player in setting up atmospheric and ocean circulation patterns worldwide.

For oceanographer Miles McPhee and his colleagues working with SHEBA experiments here at the makeshift "Ocean City," detailed measurements of heat flow in the top 30 meters of ocean will supply critical pieces for solving the Arctic climate puzzle. This top layer is known as the ocean's boundary layer, the zone most heavily influenced by what occurs at the ocean's surface.

"The ocean's role lies in its capability to store heat," explains Dr. McPhee as we sit in Ocean City's largest structure - located on the ice off the starboard side of the ice breaker that is our "hotel." "In fact," he continues, "the top 2 meters of ocean has the heat-carrying capacity of the entire atmosphere."

During the summer months, when the sunlight is strongest, the Arctic Ocean is storing heat. As the summer wanes and air temperatures cool, the atmosphere begins drawing more heavily on the ocean's heat account. Soon, ice forms as the water near the surface loses enough heat to reach its freezing point.

As air temperatures continue to chill, the ocean continues to transfer heat, which is stored in a latent form in the ice. As ice spreads or thickens into floes, the salt that was in the once-liquid water is expelled, forming a layer of very saline water beneath the frosty cap. This cold, dense water sinks, setting up turbulence as it does. Storms also impart turbulence to the boundary layer as they drive the ice floes across the sea surface. Thus, the boundary layer gets thoroughly mixed, displaying a uniform temperature and density.

This uniform mixing shows up clearly in temperature and electrical conductivity data (used to determine salinity, then density) displayed on McPhee's computer screen. Other devices he and his team have deployed through holes in the ice give direct readings on the strength of the turbulence within the boundary layer.

"We know a lot about the ocean boundary layer," McPhee says. "If you tell me the temperature, salinity, and how fast the ice is moving, I can give you heat flux."

What's lacking are detailed records of how that flux changes throughout the year here, data that climate modelers badly need.

McPhee says he's been gathering data here only 14 days, but he's already seeing some interesting features. The boundary layer is much less salty than he expected, suggesting that the ice melt last summer was more extensive than in the past, releasing more fresh water into the system. The notion of a severe summer melt is borne out by the difficulty the SHEBA expedition had in finding a suitably thick ice floe to serve as its base of operation.

In the past, he says, researchers seeking to set up ice stations at this time of year had few problems finding ice floes 3 or more meters thick. Summer melting might reduce the floe's thickness by 60 or 70 centimeters. Our floe is only about 2 meters (about 78 inches) thick at its thickest. Some equipment is set up on ice half a meter thick.

McPhee's display shows a sharp temperature spike just below the boundary layer - the leftovers from summer's warming. It's only about 0.6 degree C warmer than the boundary layer above it, but "that's a real surprise to us," he says. "That's a significant amount of heat. It's enough to melt 80 or 90 centimeters of ice over a summer."

"I truly think it is getting warmer in the Arctic," he says. "The thickness of the ice is an obvious signal."

One thing he'll be looking for this winter, he says, is how thoroughly storm-generated turbulence mixes that leftover warmer water into the boundary layer, which he expects to extend to 45 to 50 meters during the winter.

"This is the best location in the world for studying ocean boundary layers," he says.